Field of Endeavor
The present application relates to proppants for hydraulic fracturing and more particularly to encapsulated proppants for hydraulic fracturing.
State of Technology
Hydrofracturmg for oil, gas, and geothermal recovery is in wide use particularly for low permeability reservoirs such as shales. In these cases, large hydraulic pressure is applied to a rock to cause a fracture to penetrate the rock, allowing access to extract the heat, oil, or gas. The fracture typically would close on it's own after the release of the hydraulic pressure pulse, and must be “propped” with a granular material that holds the fracture open.
The transport of proppant to the relatively small fracture is a key issue in designing hydraulic fractures. The proppant must fill the fracture (appropriately sized), it must hold the fracture open (strong enough), and it must not generate additional fine material either through damaging the rock, or by breaking down itself (the fine material can block flow). These properties typically require material such as very-well-rounded sand, epoxy coated sands, or even sintered ceramics to be used as proppant. All of these materials, however, are relatively dense, and typically get more dense as more strength is required. That density requires the fracturing fluid to be very viscous in order to transport the proppant completely into the fracture, and typically a considerable excess of water is used to fully drive the proppant to the fracture limits. The highly altered chemistry (for viscosity) and the excess amounts lead to increased expense in fracturing operations, and cause many of the observed environmental issues in shale gas fracturing operations.
Applicants have develop technology for carbon dioxide capture that encapsulates a reactive chemical (in that case a solvent) within a permeable polymer shell that keeps the reactive chemical encapsulated, while permitting some reactants (water and carbon dioxide) to pass through and enter the solvent. The present invention uses that same basic technology to encapsulate the reactive chemicals for creating a strong proppant, keeping them from reacting until the appropriate time. The silicone-based capsules already tested will be appropriate for proppant use, as they are strong, non-reactive, and allow water permeation required to initiate hardening of the reactive proppant precursors carried inside.